Water ingestion provides an early signal inhibiting osmotically stimulatedvasopressin secretion in rats

Dehydrated dogs are known to inhibit secretion of vasopressin (VP) within m inutes after drinking water, before plasma osmolality (P-osmol) diminishes. The present studies determined whether water ingestion causes a similar ra pid inhibition of neurohypophyseal hormone secretion in rats. Adult rats we re infused with 1 M NaCl (2 ml/h iv) for 240 min to stimulate VP and oxytoc in (OT) secretion. After 220 min of infusion, rats were given water to drin k for 5 min, and blood samples were taken 5 and 15 min later for RIA. Plasm a VP (pVP) was much lower when rats ingested water than when they drank not hing even though P-osmol was not significantly altered. Plasma OT (pOT) was affected similarly. In contrast, no effects on pVP or pOT occurred when ra ts drank isotonic NaCl solution for 5 min in amounts comparable to the wate r intakes (similar to 5.5 ml). These results suggest that neurohypophyseal secretion of VP and OT in rats is inhibited rapidly by water drinking, and that this inhibition is mediated by a visceral signal of osmotic dilution r ather than by the act of drinking per se.